1. Field of the Invention
The present invention relates to a semiconductor memory device. More specifically, it relates to a semiconductor memory device characterized by a resistance change element, which varies in resistance due to the direction in which a voltage is applied, and a read-out circuit and a write-in circuit configured with a common small circuit.
2. Description of the Related Art
Elements capable of changing their own resistances and thereby providing two or more resistance values have been suggested. A magneto resistive random access memory (MRAM) having a magnetic tunnel junction (MTJ) device functions based on spin of the material, a chalcogenide such as Ge2Sb2Te5 in which phase change can be utilized, a solid electrolyte such as Cu2S or Ag-rich Ag—Ge—Se based chalcogenide in which ionic conduction is utilized, a device that has an organic material such as a metallic layer sandwiched between insulating films, or Rotaxane or other molecular materials, or a colossal magneto resistive (CMR) material such as Pr0.7Ca0.3MnO3, and a device having an oxide have been suggested for such elements. The above materials can function as a memory element by assigning data to respective differing resistances.
Utilization of ionic conduction in a solid electrolyte such as Cu2S has been disclosed in, for example, “REPRODUCIBLE CURRENT SWITCHING IN COPPER SULFIDE FILMS”, written by T. Sakamoto, et. al, Extended Abstract of the 2002 International Conference on Solid State Devices and Materials, Nagoya, 2002, p. 264–265, and ‘A NONVOLATILE PROGRAMMABLE SOLID ELECTROLYTE NANOMETER. SWITCH’, written by T. Sakamoto, et. al, 2004 IEEE International Solid-state Circuits Conference, session 16, TD: Emerging Technologies and Circuits, 16.3.
Resistance change elements other than MTJ elements serve as two-terminal elements. The difference in resistance of some resistance change elements is based on the direction in which voltage is applied. Since resistance change elements are two-terminal elements, those elements can be densely formed. In addition, usage of a material other than silicon for those resistance change elements allows formation of elements in an interconnect layer. This may allow reduction of the entire circuit area.
On the other hand, since each of those resistance change elements have two terminals, a method of reading a current value flowing through an element as a voltage is applied is conventionally used as an information read-out method. For example, a method of reading out the resistances of elements in a memory cell array, which is arranged at respective cross-points, using a charge-injection sense amplifier has been disclosed in Japanese Patent Application Laid-Open No. 2003-323791. However, typically, such circuit is relatively large. In addition, since such circuit is a read-only circuit, a write-in circuit must be provided independently. This creates a problem that the circuit increases in size.
The present invention allows configuration of a small circuit commonly used as a read-out circuit and a write-in circuit of a semiconductor memory device, which includes a resistance change element that varies in resistance due to the direction in which a voltage applied.